WASHINGTON — Cutting dietary fat may also cut the risk of ovarian cancer, says a study of almost 40,000 older women that found the first hard evidence that menu changes protect against this particularly lethal cancer.

But don’t wait too long to get started: The protection didn’t kick in until the women had eaten less fat for four years and counting.

Until now, the only known prescription against ovarian cancer — aside from surgically removing the ovaries — was to use birth control pills. Use for five years can lower the cancer risk by up to 60 percent, protection that lingers years after pill use ends.

The new findings now offer an option for post-menopausal women to try as well.

Those who followed a low-fat diet for eight years cut their chances of ovarian cancer by 40 percent, researchers reported Tuesday in the Journal of the National Cancer Institute.

“This is really good news,” said Dr. Jacques Rossouw of the National Institutes of Health, which funded the work. “But you have to stick with the diet.”

It’s arguably the most promising finding of the mammoth Women’s Health Initiative dietary study, which enrolled tens of thousands of healthy women ages 50 to 79 to track the role of fat in several leading killers. Some women were assigned to cut the total fat in their diets to 20 percent of calories — from an average of 35 percent — while others continued their usual diets for comparison.

Yet the study so far has found the diet made little impact on rates of breast cancer, colorectal cancer and heart disease. There are a number of theories: Maybe the women started healthier eating too late; most were overweight, a major risk factor, and the diet wasn’t designed to shed pounds. Nor did most women actually cut enough fat.

But despite all those hurdles, a low-fat diet did appear protective against ovarian cancer — and the women who started with the worst diets and cut fat the most got the most benefit.

Ovarian cancer is fairly rare, affecting one in 60 women compared with the one in 9 who will get breast cancer. But it is among the grimmest of diagnoses, because ovarian cancer usually is detected only after it has spread throughout the abdomen, making it much harder to treat. Only 45 percent of patients survive five years.

The American Cancer Society estimates that 22,430 U.S. women will be diagnosed with ovarian cancer this year; 15,280 women will die of it.

Ovarian cancer can strike anytime in adulthood, but risk increases with age. Mutations in the so-called breast cancer genes BRCA1 and BRCA2 also increase the risk of ovarian cancer — and women in the new study have not yet been tested for those genes, to see if the low-fat diet proves more or less beneficial for them.

Why would diet affect ovaries? The theory is that fat intake increases the amount of estrogen circulating in the blood, which may in turn overstimulate sensitive ovaries.

Indeed, blood tests showed study participants on the low-fat diet experienced a 15 percent reduction in estradiol, a key form of estrogen, while non-dieters experienced no change, said study co-author Dr. Ross Prentice of Seattle’s Fred Hutchinson Cancer Research Center.

“It’s quite noteworthy,” Prentice said of the ovarian protection. “We’re really pleased to have something positive to say to American women — that undertaking a low-fat diet likely reduces your risk of ovarian cancer and perhaps other cancers as well.”

Estrogen plays a role in breast cancer, too. Yet when researchers last year checked women in this same study, they found only a 9 percent drop in breast cancer risk, not quite large enough to be sure it wasn’t due to chance. But even then, the women who cut the most fat fared better — just like with the new ovarian cancer data.

Most of the dieters cut their fat intake to 24 percent of calories, not quite as much as recommended. And over time, the fat crept back: Eight years later, they were up to 29 percent — still lower than the average American diet, noted Rossouw, of NIH’s National Heart, Lung and Blood Institute.

“It’s feasible,” he said of the diet. And, “once there is news that this does work, it may be easier to motivate people to do.”

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